ABSTRACT
The growth of autonomous intercomputer networks has led to a
desire on the part of their respective proprietors to "gateway"
from one to the other. Unfortunately, however, the implications
and shortcomings of gateways which must translate or map between
differing protocol suites are not widely understood. Some
protocol sets have such severe functionality mismatches that
proper T/MG's cannot be generated for them; all attempts to mesh
heterogeneous suites are subject to numerous problems, including
the introduction of "singularity points" on logical connections
which would otherwise be able to enjoy the advantages of
communications subnetwork alternate routing, loss of
functionality, difficulty of Flow Control resolution, higher cost
than non-translating/mapping Gateways, and the necessity of
re-creating T/MG's when a given suite changes. The preferability
of a protocol-compatible internet is also touched upon, as is the
psychology of those soi-disant architects who posit T/MG's.
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Gateways, Architectures, and Heffalumps
M. A. Padlipsky
In our collective zeal to remain (or become) abreast of the
State of the Art, we sometimes fall into one or the other (or
both) of a couple of pitfalls. Only one of these pitfalls is
particularly well-known: "Buzzwords" -- and even here merely
knowing the name doesn't necessarily effect a spontaneous
solution. The other deserves more attention: inadequate
familiarity with The Relevant Literature.
The key is the notion of what's really relevant. Often,
it's the Oral Tradition that matters; published papers, in their
attempts to seem scholarly, offer the wrong levels of abstraction
or, because of the backgrounds of their authors, are so
ill-written as to fail to communicate well. Sometimes, however,
that which is truly relevant turns out to be unfindable by a
conventional literature searcher because it isn't "in" the field
of search.
I wandered into an instructive case in point recently, when
it took me over an hour to convince a neophyte to the mysteries
of intercomputer networking (who is quite highly regarded in at
least one other area of computer science, and is by no means a
dummy) that a particular Local Area Network architecture proposal
which casually appealed to the notion of "gatewaying" to three or
four other networks it didn't have protocols in common with was a
Very Bad Thing. "Gateways" is, of course, another one of those
bloody buzzwords, and in some contexts it might have been enough
just to so label it. But this was a conversation with a bright
professional who'd recently been reading up on networks and who
wanted really to understand what was so terrible.
So I started by appealing to the Oral Tradition, pointing
out that in the ARPA internetworking research community (from
which we probably got the term "Gateway" in the first place --
and from which we certainly get the proof of concept for
internets) it had been explicitly decided that it would be too
hard to deal with connecting autonomous networks whose protocol
sets differed "above" the level of
Host-to-Communications-Subnetwork-Processor protocol. That is,
the kind of Gateway we know how to build -- and, indeed, anything
one might call a Gateway -- attaches to two (or more) comm
subnets as if it were a Host on each, by appropriately
interpreting their respective H-CSNP protocols and doing the
right things in hardware (see Figure 1), but for ARPA Internet
Gateways each net attached to is assumed to have the same
Host-Host Protocol (TCP/IP, in fact
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RFC 875 September 1982
or, anyway, IP and either TCP or some other common-to-both-nets
protocol above it), and the same process level protocols (e.g.,
Telnet, FTP, or whatever). The reason for this assuming of
protocol set homogeneity is that they "knew" the alternative was
undesirable, because it would involve the translation or mapping
between different protocol sets in the Gateways and such T/MG's
were obviously to be avoided.
Well, that didn't do the trick. "Why is a T/MG a Bad
Thing?" he wanted to know. "Because of the possibility of
irreconcilable mismatches in functionality." "For instance?"
"Addressing is the most commonly cited." "Addressing?"
Assuming the reader is as bored as I am with the dialogue
bit, I'll try to step through some specifics of the sorts of
incompatibility one can find between protocol sets in a less
theatric manner. Note that the premise of it all is that we
don't want to change either pre-existing protocol set. Let's
assume for convenience that we are trying to attach just two nets
together with a T/MG, and further assume that one of the nets
uses the original ARPANET "NCP" -- which consists, strictly
speaking, of the unnamed original ARPANET Host-Host Protocol and
the unfortunately named "1822", or ARPANET Host-IMP Protocol --
and the other uses TCP/IP.
Host addressing is the most significant problem. NCP-using
hosts have "one-dimensional" addresses. That is, there's a field
in the Host-IMP "leader" where the Host number goes. When you've
assigned all the available values in that field, your net is full
until and unless you go back and change all the IMP's and NCP's
to deal with a bigger field. Using IP, on the other hand,
addresses of Hosts are "two-dimensional". That is, there's an IP
header field in which to designate the foreign network and
another field in which to designate the foreign Host. (The
foregoing is a deliberate oversimplification, by the way.) So if
you wanted a Host on an NCP-based net to communicate with a Host
on another, TCP-based net you'd have a terrible time of it if you
also didn't want to go mucking around inside of all the different
NCP implementations, because you don't have a way of expressing
the foreign address within your current complement of addressing
mechanisms.
There are various tricks available, of course. You could
find enough spare bits in the Host-IMP leader or Host-Host header
perhaps, and put the needed internet address there. Or you could
change the Initial Connection Protocol, or even make the internet
address be the first thing transmitted as "data" by the User side
of each process-level protocol. The common failing of all such
ploys is that you're changing the pre-existing protocols, though,
and if
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RFC 875 September 1982
that sort of thing were viewed with equanimity by system
proprietors you might as well go the whole hog and change over to
the new protocol set across the board. Granted, that's a big
jump; but it must be realized that this is just the first of
several problems.
(It is the case that you could get around the addressing
problem by having the T/MG become more nearly a real Host and
terminate the NCP-based side in an application program which
would "ask" the user what foreign Host he wants to talk to on the
TCP-based side -- at least for Telnet connections. When there's
no user around, though, as would be the case in most file
transfers, you lose again, unless you fiddle your FTP. In
general, this sort of "Janus Host" -- after the Roman deity with
two faces, who was according to some sources the god of gateways
(!) -- confers extremely limited functionality anyway; but in
some practical cases it can be better than trying for full
functionality and coming up empty.)
Then there's the question of what to do about RFNM's. That
is, NCP's follow the discipline of waiting until the foreign IMP
indicates a Ready for Next Message state exists before sending
more data on a given logical connection, but if you're talking to
a T/MG, its IMP is the one you'll get the RFNM from (the real
foreign Host might not even be attached to an IMP). Now, I've
actually seen a proposal that suggested solving this problem by
altering the T/MG's IMP to withhold RFNM's, but that doesn't make
me think it's a viable solution. At the very least, the T/MG is
going to have to go in for buffering in a big way (see Figure 2).
In a possible worst case, the foreign net might not even let you
know your last transmission got through without changing its
protocols.
Going beyond the NCP-TCP example, a generic topic fraught
with the peril of functionality mismatch is that of the
Out-of-Band Signal. (There are some who claim it's also an
NCP-TCP problem.) The point is that although "any good Host-Host
protocol" should have some means of communicating aside from
normal messages "on" logical connections, the mechanizations and
indeed the semantics of such Out-of-Band Signals often differ.
The fear is that the differences may lead to incompatibilities.
For example, in NCP the OOBS is an Interrupt command "on" the
control link, whereas in TCP it's an Urgent bit in the header of
a message "on" the socket. If you want Urgent to be usable in
order to have a "virtual quit button", the semantics of the
protocol must make it very clear that Urgent is not merely the
sort of thing the NBS/ECMA Host-Host protocol calls "Expedited
Data". If, that is, the intent of the mechanism is to cause the
associated process/job/task to take special action rather than
merely the associated protocol interpreter (which need not be
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RFC 875 September 1982
part of the process), you'd better say so -- and none of the
ISO-derived protocols I've seen yet does so. And there's not
much a T/MG can do if it gets an NCP Interrupt on a control
link, notices a Telnet Interrupt Process control code on the
associated socket, and doesn't have anything other than
Expediting Data to do with it on its other side. (Expedited
Data, it may be noted, bears a striking resemblance to taking an
SST across the Atlantic, only to find no one on duty in the
Customs shed -- and the door locked from the other side.)
Functionality mismatch is not, of course, limited to
Host-Host protocols. Indeed, the following interesting situation
was observed at University College London: In their "Terminal
Gateway", which translates/maps ARPANET Telnet and "Triple X"
(CCITT X.25, X.28, X.29), they were able to get data across, as
might be expected, but only one option (echoing), which is rather
worse than might be expected. (And the UCL people are quite
competent, so the problem almost certainly doesn't have to do
with inadequate ingenuity.)
It could be argued that the real problem with Expedite Data
and Triple X is that some protocol sets are a lot worse than
others. I wouldn't dispute that. But it's still the case, to
re-use a Great Network One-liner, that:
sometimes, when you try to turn an apple into an
orange, you get back a lemon.
Nor is the likelihood of encountering irresolvable
functionality mismatches the only technical shortcoming of
Translating/Mapping Gateways. A somewhat subtle but rather
fascinating point arises if we ask what happens when traffic is
heavy enough to warrant more than one T/MG between a given pair
of protocol-incompatible nets (or even if we'd like to add some
reliability, regardless of traffic). What happens, if we think
about it a little, is a big problem. Suppose you actually could
figure out a way to translate/map between two given sets of
protocols. That would mean that for each logical connection you
had open, you'd have a wealth of state information about it for
each net you were gatewaying. But "you" now stand revealed as a
single T/MG -- and your clone next door doesn't have that state
information, so any logical connection that started its life with
you has to spend its life with you, in a state of perpetual
monogamy, as it were. Naturally, this epoxied pair-bonding could
perhaps be dealt with by still another new protocol between
T/MG's, but it's abundantly clear that there will be no easy
analogue to no-fault divorce. That is, to put it less
metophorically, it becomes at best extremely complex to do
translating/mapping at more
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RFC 875 September 1982
than one T/MG for the same logical connection. As with the
broader issue of reconciling given protocol sets at all, doing so
at multiple loci of control may or may not turn out to be
feasible in practice and certainly will be a delicate and complex
design task.
One more NCP/TCP problem: When sending mail on an NCP-based
net, the mail (actually, File Transfer) protocol currently only
uses the addressee's name, because the Host was determined by the
Host-Host Protocol. If you're trying to get mail from an
NCP-based net to a TCP-based net, though, you're back in the Host
addressing bind already discussed. If you don't want to change
NCP (which, after all, is being phased out), you have to do
something at the process level. You can, but the "Simple Mail
Transfer Protocol" to do it takes 62 pages to specify in ARPANET
Request for Comments 788.
If things get that complicated when going from NCP to TCP,
where there's a close evolutionary link between the Host-Host
protocols, and the process-level protocols are nominally the
same, what happens when you want to go from DECNET, or from SNA,
or from the as-yet incomplete NBS or ISO protocol sets? There
may or may not turn out to be any aspects that no amount of
ingenuity can reconcile, but it's abundantly clear that
Translating/Mapping Gateways are going to have to be far more
powerful systems than IP Gateways (which are what you use if both
nets use the same protocol sets above the Host to Comm Subnet
Processor protocol). And you're going to need a different T/MG
for each pair of protocol sets. And you may have to tinker with
CSNP internals.... An analogy to the kids' game of Telephone (or
Gossip) comes to mind: How much do you lose each time you
whisper to your neighbor who in turn whispers to the next
neighbor? What, for that matter, if we transplant the game to
the United Nations and have the whisperers be translators who
have speakers of different languages on each side?
Other problem areas could be adduced. For example, it's
clear that interpreting two protocol sets rather than one would
take more time, even if it could be done. Also, it should be
noted that the RFNM's Problem generalizes into a concern over
resolving Flow Control mismatches for any pair of protocol sets,
and could lead to the necessity of having more memory for buffers
on the T/MG than on any given Host even for those cases where
it's doable in principle. But only one other problem area seems
particularly major, and that is the old Moving Target bugaboo:
For when any protocol changes, so must all the T/MG's involving
it, and as there have already been three versions of SNA,
presumably a like number of versions of DECNET, and as there are
at least two additional levels which ISO should be acknowledging
the existence of, the fear of having to re-do T/MG's should serve
as a considerable deterrent to doing them
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RFC 875 September 1982
in the first place. (This apparent contravention of the
Padlipsky's Law to the effect that Implemented Protocols Have
Barely Finite Inertia Of Rest is explained by a brand-new
Padlipsky's Law: To The Technologically Naive, Change Equals
Progress; To Vendors, Change Equals Profit.)
At any rate, it's just not clear that a given Translating/
Mapping Gateway can even be built; you have to look very closely
at the protocol sets in question to determine even that. It's
abundantly clear that if a given one can be built it won't be
easy to do (see Figure 3). Yet "system architect" after "system
architect", apparently in good faith, toss such things into their
block diagrams. Assuming that the architectural issue isn't
resolved by a fondness for the Gothic in preference to the more
modern view that form should follow function, let's pause briefly
to visualize an immense, turreted, crenellated, gargoyled ...
microprocessor, and return to the question of why this sort of
thing happens.
It's clear that buzzwording is a factor. After all, "system
architects" in our context are usually employees of contractors
and their real role in life is not to build more stately mansions
but to get contracts, so it's not surprising to find appeal to
the sort of salesmanship that relies more heavily on fast patter
than precision. Another good analogy: I once went to one of the
big chain electronics stores in response to an ad for a cassette
recorder that "ran on batteries or house current" for $18, only
to find that they wanted an additional $9 for the (outboard) AC
adaptor. Given the complexities of T/MG's, however, in our case
it's more like an $18 recorder and a $36 adaptor.
But is buzzwording all there is? Clearly not, for as
mentioned earlier there's also ignorance of the Oral Tradition in
play. Whether the ignorance is willful or not is probably better
left unexamined, but if we're willing to entertain the notion
that it's not all a bait-and-switch job akin to the
separately-priced AC adaptor, we see that those who casually
propose T/MG's haven't done enough homework as to the real state
of the art.
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RFC 875 September 1982
What ever became of that early reference to The Relevant
Literature, though? Surely you didn't think I'd never ask. The
answers are both implied in the assertion that:
Gateways are Heffalumps
as you'll plainly see once you've been reminded of what
Heffalumps are. Dipping into The Relevant Literature, then,
let's reproduce the opening of the Heffalumps story:
One day, when Christopher Robin and Winnie-the-Pooh
and Piglet were all talking together, Christopher Robin
finished the mouthful he was eating and said carelessly:
"I saw a Heffalump today, Piglet."
"What was it doing?" asked Piglet.
"Just lumping along," said Christopher Robin.
"I don't think it saw me."
"I saw one once," said Piglet. "At least, I think
I did," he said. "Only perhaps it wasn't."
"So did I," said Pooh, wondering what a Heffalump
was like.
"You don't often see them," said Christopher Robin
carelessly.
"Not now," said Piglet.
"Not at this time of year," said Pooh.
Then they all talked about something else, until it
was time for Pooh and Piglet to go home together.
(To satisfy the lazy reader -- who'd actually be better off
searching for it in both -- it's from Winnie-the Pooh, not The House at
Pooh Corner.)
Pooh, in case you still don't recall, decides to make a Heffalump
Trap. (Piglet is sorry he didn't think of it first.) He baits it with
a jar of honey, after making sure that it really was honey all the way
to the bottom, naturally. In the middle of the night, he goes to the
Trap to get what's left of the honey and gets his head stuck in the jar.
Along comes Piglet, who sees this strange creature with a jar-like head
making frightful noises, and, having known no more than Pooh what
Heffalumps really were, assumes that a Heffalump has indeed been Trapped
and is duly terrified.
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RFC 875 September 1982
It would probably be too moralistic to wonder how much Christopher
Robin actually knew about Heffalumps in the first place. The
"Decorator", based on the picture on page 60 of my edition, clearly
thinks C.R. thought they were elephants, but I still wonder. At best,
though, he knew no more about them than the contractor did about
Gateways in the proposal that started this whole tirade off.
NOTE: FIGURE 1. Defining Characteristic of All Flavors of
Gateways, FIGURE 2. Gateway and Translating/Mapping Gateway,
Approximately to Scale, and FIGURE 3. Respective Internals Schematics,
may be obtained by writing to: Mike Padlipsky, MITRE Corporation, P.O.
Box 208, Bedford, Massachusetts, 01730, or sending computer mail to
Padlipsky@ISIA.
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